Touch sensors

ABSTRACT

A touch sensor comprises a sensor element for sensing the position of an object over a display screen in a first direction and in a second direction. The sensor element comprises a substrate having an electrode pattern disposed thereon to define an array of sensor nodes. The electrode pattern comprises a plurality of conductors arranged to follow portion of a network of lines extending generally in the first direction, wherein each of the lines follows a zigzag (saw-tooth) pattern comprising an alternating series of first line segments arranged at a first angle to the first direction and second line segments arranged at a second angle to the first direction, and wherein respective ones of the first line segments in each line are co-linear with respective ones of the first line segments in an adjacent line. Such a configuration leads to a reduced visibility of the conductors overlying the display screen.

This application is a national phase of International Application No.PCT/GB2015/052337 filed Aug. 13, 2015 and published in the Englishlanguage which claims priority to United Kingdom Patent Application No.1415829.9 filed Sep. 8, 2014, which are hereby incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to the field of touch sensors, for exampletouch sensors for overlying a display screen to provide atouch-sensitive display (touch screen). In particular, embodiments ofthe invention relate to designs for electrode patterns for such sensorsfor sensing the presence of one or more touching objects within atwo-dimensional sensing area.

A capacitive touch sensor can be generalised as one that uses a physicalsensor element comprising an arrangement of electrically conductiveelectrodes extending over a touch sensitive area (sensing area) todefine sensor nodes and a controller chip connected to the electrodesand operable to measure changes in the electrical capacitance of each ofthe electrodes or the mutual-capacitance between combinations of theelectrodes. The electrodes are typically provided on a substrate. Insome configurations electrodes are provided on both sides of asubstrate, and these may be referred to as two-sided (two-layer)designs. In other configurations electrodes are provided on a singleside of a substrate, and these may be referred to as single-sided(single-layer) designs. Single-sided designs are sometimes preferredbecause they have reduced manufacturing costs as compared tomulti-layered designs. However, single-layer designs can be morechallenging from a design point of view because of the restrictedtopology, principally because electrode interconnections cannot crossone another in a single plane.

FIG. 1 schematically shows some principal components of a generictwo-sided capacitive touchscreen comprising a physical sensor element100. The touch screen is represented in plan view (to the left in thefigure) and also in cross-sectional view (to the right in the figure).

The touch screen is configured for establishing the position of a touchwithin a two-dimensional sensing area by providing Cartesian coordinatesalong an X-direction (horizontal in the figure) and a Y-direction(vertical in the figure). In this example the sensor element 100 isconstructed from a substrate 103 that could be glass or plastic or someother insulating material and upon which is arranged an array ofelectrodes consisting of multiple laterally extending parallelelectrodes, X-electrodes 101 (row electrodes), and multiple verticallyextending parallel electrodes, Y-electrodes 102 (column electrodes),which in combination allow the position of a touch 109 to be determined.To clarify the terminology, and as will be seen from FIG. 1, theX-electrodes 101 (row electrodes) are aligned parallel to theX-direction and the Y-electrodes 102 (column electrodes) are alignedparallel to the Y-direction. Thus the different X-electrodes allow theposition of a touch to be determined at different positions along theY-direction while the different Y-electrodes allow the position of atouch to be determined at different positions along the X-direction.That is to say in accordance with the terminology used herein, theelectrodes are named (in terms of X- and Y-) after their direction ofextent rather than the direction along which they resolve position.Furthermore, the electrodes may also be referred to as row electrodesand column electrodes. It will however be appreciated these terms aresimply used as a convenient way of distinguishing the groups ofelectrodes extending in the different directions. In particular, theterms are not intended to indicate any specific electrode orientation.In general the term “row” will be used to refer to electrodes extendingin a horizontal direction for the orientations represented in thefigures while the terms “column” will be used to refer to electrodesextending in a vertical direction in the orientations represented in thefigures.

In some cases, each electrode may have a more detailed structure thanthe simple “bar” structures represented in FIG. 1, but the operatingprinciples are broadly the same. The sensor electrodes are made of anelectrically conductive material such as copper or Indium Tin Oxide(ITO). The nature of the various materials used depends on the desiredcharacteristics of the touch screen. For example, a touch screen mayneed to be transparent, in which case ITO electrodes and a plasticsubstrate are common. On the other hand a touch pad, such as oftenprovided as an alternative to a mouse in laptop computers is usuallyopaque, and hence can use lower cost copper electrodes and anepoxy-glass-fibre substrate (e.g. FR4). Referring back to FIG. 1, theelectrodes are electrically connected via circuit conductors 104 to acontroller chip 105, which is in turn connected to a host processingsystem 106 by means of a communication interface 107. The host 106interrogates the controller chip 105 to recover the presence andcoordinates of any touch or touches present on, or proximate to thesensor 103. In the example, a front cover (also referred to as a lens orpanel) 108 is positioned in front of the sensor 103 and a single touch109 on the surface of the cover 108 is schematically represented.

Note that the touch itself does not generally make direct galvanicconnection to the sensor 103 or to the electrodes 102. Rather, the touchinfluences the electric fields 110 that the controller chip 105generates using the electrodes 102. With appropriate analysis ofrelative changes in the electrodes' measured capacitance/capacitivecoupling, the controller chip 105 can thus calculate a touch position onthe cover's surface as an XY coordinate 111. The host system cantherefore use the controller chip to detect where a user is touching,and hence take appropriate action, perhaps displaying a menu oractivating some function.

There are many different material combinations and electrodeconfigurations to allow creation of a touch screen and the examplediscussed above is just one.

A further aspect of capacitive touch sensors relates to the way thecontroller chip uses the electrodes of the sensor element to make itsmeasurements. There are two main classes of controller in this regard.

A first class is based on measuring what is frequently referred to as“self-capacitance”. Reference is made to FIG. 2. In this design of acapacitive sensor, the controller 201 will typically apply someelectrical stimulus (drive signal) 202 to each electrode 203 which willcause an electric field to form around it 204. This field couplesthrough the space around the electrode back to the controller chip vianumerous conductive return paths that are part of the nearby circuitry205, product housing 206, physical elements from the nearby surroundings207 etc., so completing a capacitive circuit 209. The overall sum ofreturn paths is typically referred to as the “free space return path” inan attempt to simplify an otherwise hard-to-visualize electric fielddistribution. The important point to realise is that the controller isonly driving each electrode from a single explicit electrical terminal208; the other terminal is the capacitive connection via this “freespace return path”. The capacitance measured by the controller is the“self-capacitance” of the sensor electrode (and connected tracks)relative to free space (or Earth as it is sometimes called) i.e. the“self-capacitance” of the relevant sensor electrode. Touching orapproaching the electrode with a conductive element 210, such as a humanfinger, causes some of the field to couple via the finger through theconnected body 213, through free space and back to the controller. Thisextra return path 211 can be relatively strong for large objects (suchas the human body), and so can give a stronger coupling of theelectrode's field back to the controller; touching or approaching theelectrode hence increases the self-capacitance of the electrode. Thecontroller is configured to sense this increase in capacitance. Theincrease is strongly proportional to the area 212 of the applied touchand is normally weakly proportional to the touching body's size (thelatter typically offering quite a strong coupling and therefore notbeing the dominant term in the sum of series connected capacitances).

In a common two-layer self-capacitance sensor the electrodes arearranged on an orthogonal grid, generally with a first set of electrodeson one side of a substantially insulating substrate and the other set ofelectrodes on the opposite side of the substrate and oriented atnominally 90° to the first set. There are also structures where the gridis formed on a single side of the substrate and small conductive bridgesare used to allow the two orthogonal sets of electrodes to cross eachother without short circuiting. However, these designs are more complexto manufacture and less suitable for transparent sensors. There are alsoknown designs where the electrode pattern is formed on a single side ofa substrate and external connections are used to allow the respectiveelectrodes to be appropriately connected, as discussed further below.One set of electrodes is used to sense touch position in a first axisthat we shall call “X” and the second set to sense the touch position inthe second orthogonal axis that we shall call “Y”.

In a self-capacitance touch sensor, the controller can either drive eachelectrode in turn (sequential) with appropriate switching of a singlecontrol channel or it can drive them all in parallel with an appropriatenumber of separate control channels. In the former sequential case, anyneighbouring electrodes to a driven electrode are sometimes grounded bythe controller to prevent them becoming touch sensitive when they arenot being sensed (remembering that all nearby capacitive return pathswill influence the measured value of the actively driven electrode). Inthe case of the parallel drive scheme, the nature of the stimulusapplied to all the electrodes is typically the same so that theinstantaneous voltage on each electrode is approximately the same. Thedrive to each electrode is electrically separate so that the controllercan discriminate changes on each electrode individually, but the drivingstimulus in terms of voltage or current versus time, is the same. Inthis way, each electrode has minimal influence on its neighbours (theelectrode-to-electrode capacitance is non-zero but its influence is only“felt” by the controller if there is a voltage difference between theelectrodes).

The second class of controller is based on measuring what is frequentlyreferred to as “mutual-capacitance”. Reference is made to FIG. 3. Inthis design of a capacitive sensor the controller 301 will sequentiallystimulate each of an array of transmitter (driven/drive) electrodes 302that are coupled by virtue of their proximity to an array of receiverelectrodes 303. The resulting electric field 304 is now directly coupledfrom the transmitter to each of the nearby receiver electrodes; the“free space” return path discussed above plays a negligible part in theoverall coupling back to the controller chip when the sensor is notbeing touched. The area local to and centred on the intersection of atransmitter and a receiver electrode is typically referred to as a“node”. Now, on application or approach of a conductive element 305 suchas a human finger, the electric field 304 is partly diverted to thetouching object 305. An extra return path to the controller 301 is nowestablished via the body 306 and “free-space” in a similar manner tothat described above. However, because this extra return path acts tocouple the diverted field directly to the controller chip 301, theamount of field coupled to the nearby receiver electrode 303 decreases.This is measured by the controller chip 301 as a decrease in the“mutual-capacitance” between that particular transmitter electrode andreceiver electrodes in the vicinity of the touch. The controller sensesthis change in capacitance of one or more nodes. For example, if areduction in capacitive coupling to a given Y-electrode is observedwhile a given X-electrode is being driven, it may be determined there isa touch in the vicinity of where the given X-electrode and givenY-electrode cross within the sensing surface. The magnitude of acapacitance change is nominally proportional to the area 307 of thetouch (although the change in capacitance does tend to saturate as thetouch area increases beyond a certain size to completely cover the nodesdirectly under the touch) and weakly proportional to the size of thetouching body (for reasons as described above). The magnitude of thecapacitance change also reduces as the distance between the touch sensorelectrodes and the touching object increases.

In a common two-sided mutual-capacitance sensor the transmitterelectrodes and receiver electrodes are arranged as an orthogonal grid,with the transmitter electrodes on one side of a substantiallyinsulating substrate and the receiver electrodes on the opposite side ofthe substrate. This is as schematically shown in FIG. 3. In FIG. 3 afirst set of transmitter electrodes 303 is shown on one side of asubstantially insulating substrate 308 and a second set of receiverelectrodes 302 is arranged at nominally 90° to the transmitterelectrodes on the other side of the substrate. There are also structureswhere the grid is formed on a single side of the substrate and smallinsulating bridges, or as discussed below external connections, are usedto allow the transmitter and receiver electrodes to be connected to inrows and columns without short circuiting.

By using interpolation between adjacent nodes for both types ofcapacitive touch sensor a controller chip can typically determine touchpositions to a greater resolution than the spacing between electrodes.Also there are established techniques by which multiple touches within asensing area, and which might be moving, can be uniquely identified andtracked, for example until they leave the sensing area.

For a touch sensor that is transparent, for example because it isintended to overlay a display the electrodes may be provided by atransparent conductor material, such as ITO. However, using transparentconductor materials can be relatively expensive, for example as comparedto using copper. Accordingly, it has been proposed to define electrodesfor a transparent conductor using a mesh of thin copper traces.

FIG. 4 schematically represents an arrangement of electrodes 34 on oneside of a two-sided sensor 30, for example of the kind represented inFIGS. 1, 2 and 3, and in which the electrodes are defined by a mesh ofthin copper traces on a substrate 32. Each electrode 34 is defined by anappropriately-shaped region (in this case horizontal bars) comprising agrid of copper wires. In this example the copper wire grid is angled ataround 45 degrees to the horizontal/vertical directions and is made ofcopper wires having a width on the substrate of around 3 microns and apitch of around 200 microns (the thickness of the copper layer willtypically be around 0.1 to 2 microns). The touch sensor is overalltransparent because of the copper wires cover only a relatively smallfraction of the area. Furthermore, with relatively large area electrodes(i.e. large compared to the 200 microns pitch of the wire mesh) such asrepresented in FIG. 4, the wire mesh is barely visible to a user lookingthrough the touch sensor at an underlying screen. That is to say, thewire mesh does not give rise to significant visual artefacts.

FIG. 5 schematically represents a conventional single-sided(single-layer) electrode pattern for a capacitive touch sensor 40. Thesensor 40 comprises an array of sensing nodes 42 arranged in a pluralityof rows and columns across a two-dimensional sensing surface. In thisexample there are five rows schematically labelled R1 to R5 (runninghorizontally for the orientation represented in the figure) and sixcolumns schematically labelled C1 to C6 (running vertically for theorientation represented in the figure). Thus the sensing surface extendshorizontally from a first (left) edge 47A adjacent column C1 to a second(right) edge 47B adjacent column C6 and extends vertically from a third(top) edge 47C adjacent row R1 to a fourth (bottom) edge 47D adjacentrow R5.

Each sensing node 42 comprises a first electrode 43 and a secondelectrode 44. The first electrodes are schematically represented in FIG.5 with darker shading than the second electrodes. A plurality of traces45 connect respective ones of the first electrodes 43 to a perimeter ofthe sensing surface, in this case down to the fourth (bottom) edge 47Dadjacent row R5. There is a separate trace 45 for each of the firstelectrodes 43. The respective first electrodes 43 of each row R1 to R5are electrically connected together outside the surface of the sensingarea by external wiring (not shown) connecting to the respective traces45 at the perimeter of the sensing area. A plurality of further traces46 interconnect respective ones of the second electrodes 44 in the samecolumn, and the respective further traces also extend down to theperimeter of the sensing area along the fourth edge 47D. Ground traces48 (schematically represented in FIG. 5 with dotted lines) are providedat locations where traces 45 connecting to the first electrodes 43 andfurther traces 46 connecting to the second electrodes 46 would otherwisebe adjacent.

Thus, in the arrangement represented in FIG. 5 the first electrodes 43in each row are interconnected (via their respective traces 45 andexternal wiring) and the second electrodes in each column areinterconnected (by the further traces 46) within the sensing area. Inthis regard the arrangement of electrodes in FIG. 5 provides an array ofinterconnected rows and columns defining a two-dimensional array ofsensing nodes. In effect the sensing nodes 43 of FIG. 5 correspond tothe sensing nodes at the crossing points in the two-layer designs ofFIGS. 1 to 3, but with electrodes provided on only a single layer of asubstrate. Thus, the approach of FIG. 5 can be advantageous in certaincircumstances, for example because of simpler manufacturing and/orhigher transparency. The sensing element represented in FIG. 5 can beconnected to conventional drive circuitry for establishing the positionof an object adjacent the sensing surface in accordance withconventional techniques such as discussed above with reference to FIGS.1 to 3. Thus the sensing element can be used in a mutual-capacitancemode in which capacitive coupling between the respective firstelectrodes and the second electrodes are measured to identify whichsensing nodes are associated with a change in mutual capacitance causedby a proximate object. The sensing elements can also be used in aself-capacitance mode in which the self-capacitance of the respectiveelectrodes are separately measured to identify which sensing nodes areassociated with a change in mutual capacitance. In this regard, theinterconnection of the electrodes into rows and columns provides amatrix approach which reduces the number of control channels required(as compared to approaches where the individual sensing nodes arecoupled to separate measurement channels).

Whilst a single-layer design of the type represented in FIG. 5 can beadvantageous from a manufacturing point of view, it requires relativelythin trace circuitry to avoid overly-large areas of insensitivitybetween columns. For example, the traces 45 connecting from the loweredge to the respective first electrodes 43 of the respective sensornodes 42 in a given column in a typical implementations may be separatedby only 100 microns or so. It is not possible to provide a group of suchclosely spaced electrodes using a wire-mesh arrangement of the kindrepresented in FIG. 4 with a pitch of around 200 microns, and simplyusing a regular grid having a smaller pitch size can be expected toreduce transparency to levels which are unacceptable for manytouch-screen applications.

An alternative approach could be to simply replicate the pattern of FIG.5 with linear electrodes, i.e. using an array of thin copper electrodesdeposited on a substrate and extending generally in the verticaldirection (for the orientation of in FIG. 5) with appropriate horizontalconnections for defining the pattern as appropriate. However, theInventor has recognised drawbacks with this approach in situations wherethe sensor is to overlay a display screen (i.e. where the positionsensor is incorporated in a touchscreen). In particular, the Inventorhas recognised the way in which the electrodes extending generally inthe vertical direction of FIG. 5 interact with pixels in the underlyingdisplay screen can give rise to distracting effects such as moirépatterning and variations in apparent colour, both across the screen andfor different viewing angles, as the electrodes obscure differentportions of the underlying pixels.

There is therefore a desire to provide touch sensors with electrodepatterns that can be implemented using non-transparent conductors in asingle layer design with reduced visible artefacts.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a sensorelement for sensing the position of an object in a first direction andin a second direction, the sensor element comprising: a substrate havingan electrode pattern disposed thereon, wherein the electrode patterncomprises a plurality of conductors selectively arranged on a network oflines extending generally in the first direction, wherein each of thelines follows a zigzag pattern comprising an alternating series of firstline segments arranged at a first angle to the first direction andsecond line segments arranged at a second angle to the first direction,and wherein respective ones of the first line segments in each line areco-linear with respective ones of the first line segments in an adjacentline and respective ones of the second line segments in each line arenot co-linear with respective ones of the second line segments in anadjacent line.

In accordance with certain embodiments the length of the first linesegments is different from the length of the second line segments.

In accordance with certain embodiments the length of the first linesegments is greater than the length of the second line segments.

In accordance with certain embodiments the angle between respective onesof the first line segments and the first direction is less than an angleselected from the group comprising: 45 degrees; 40 degrees; 35 degrees;30 degrees; 25 degrees; 20 degrees; 15 degrees and 10 degrees.

In accordance with certain embodiments the angle between respective onesof the second line segments and the first direction is greater than anangle selected from the group comprising: 45 degrees; 50 degrees; 55degrees; 60 degrees; 65 degrees; 70 degrees; 75 degrees and 80 degrees.

In accordance with certain embodiments the angle between the first linesegments and the second line segments is around 90 degrees.

In accordance with certain embodiments the joins between first linesegments and second line segments in one line are at the same locationsalong the first direction as the joins between first line segments andsecond line segments in an adjacent line.

In accordance with certain embodiments the electrode pattern furthercomprises connector portions arranged to selectively connect conductorsarranged on adjacent lines of the network of lines together.

In accordance with certain embodiments the connector portions arearranged to have an approximately uniform density across the sensorelement.

In accordance with certain embodiments the electrode pattern furthercomprises gaps arranged to selectively separate conductors arranged onthe same line of the network of lines from each other.

In accordance with certain embodiments the gaps are arranged to have anapproximately uniform density across the sensor element.

In accordance with certain embodiments, the sensor element furthercomprises a controller coupled to respective ones of the conductors andarranged to measure changes in a capacitive coupling associated with theconductors.

In accordance with certain embodiments the controller is furtheroperable to determine the position of an object based on the measuredchanges in the capacitive coupling associated with the conductors.

According to a second aspect of the invention there is provided a touchscreen comprising the sensor element of the first aspect of theinvention and a display screen, wherein the electrode pattern isarranged over the display screen.

In accordance with certain embodiments the display screen comprises anarray of pixels arranged in columns extending along the first directionand rows extending along the second direction, and wherein each pixel isseparated along the second direction into a plurality of sub-pixels forrepresenting different colours of the display screen.

In accordance with certain embodiments the combined extent of one firstline segment and one second line segment along the first direction is oris approximately an integer multiple of the extent of one of the pixelsin the first direction.

In accordance with certain embodiments the separation between adjacentlines along the second direction is or is approximately an integermultiple of the extent of one of the sub-pixels in the second direction.

In accordance with certain embodiments the separation between adjacentlines along the second direction is or is approximately an integermultiple of the extent of one of the sub-pixels in the second directionthat is different from the number of sub-pixels in a pixel.

According to a third aspect of the invention there is provided a methodof manufacturing a sensor element for sensing the position of an objectin a first direction and in a second direction, the method comprising:disposing an electrode pattern on a substrate, wherein the electrodepattern comprises a plurality of conductors arranged on a network oflines extending generally in the first direction, wherein each of thelines follows a zigzag pattern comprising an alternating series of firstline segments arranged at a first angle to the first direction andsecond line segments arranged at a second angle to the first direction,and wherein respective ones of the first line segments in each line areco-linear with respective ones of the first line segments in an adjacentline and respective ones of the second line segments in each line arenot co-linear with respective ones of the second line segments in anadjacent line.

It will be appreciated that features and aspects of the inventiondescribed above in relation to the first and other aspects of theinvention are equally applicable to, and may be combined with,embodiments of the invention according to other aspects of the inventionas appropriate, and not just in the specific combinations describedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described by way of example only with reference tothe following drawings in which:

FIG. 1 schematically illustrates a typical touchscreen/touch sensorsystem;

FIG. 2 schematically illustrates a typical self-capacitance typetouchscreen system;

FIG. 3 schematically illustrates a typical mutual-capacitance typetouchscreen system;

FIG. 4 schematically illustrates an approach for providing electrodesfor a touch sensor using a wire mesh;

FIG. 5 schematically illustrates a conventional single-sided electrodepattern for a two-dimensional capacitive sensor;

FIG. 6 is a schematic representation of a portion of display screen ofthe kind often used in conjunction with capacitive touch sensors toprovide a touch sensitive display screen;

FIGS. 7, 8 and 9 schematically illustrates some aspects of asingle-sided electrode pattern for a two-dimensional capacitive sensorin accordance with certain embodiments of the invention;

FIG. 10 schematically shows some components of a touch sensor accordingto an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 6 is a schematic representation of a portion of display screen 60of the kind often used in conjunction with capacitive touch sensors toprovide a touch sensitive display screen. As is well established, thedisplay screen 60 comprises a plurality of pixels 62. The portion of thedisplay 60 represented in FIG. 6 comprises a six-by-six array of pixels62. It will be appreciated the display screen will typically contain alarger number of pixels extending over a larger area in accordance withconventional display screen technologies. The display screen 60represented in FIG. 6 is a colour-screen and each pixel 62 comprisesthree differently coloured sub-pixels. In this example the screen isbased on RGB colours and so each pixel comprises a red sub-pixel 64R, agreen sub-pixel 64G and a blue sub-pixel 64B, as schematicallyrepresented in the figure. It will be appreciated the display screen mayoperate in accordance with any established display screen technology,for example based on a thin film transistor display technology.

It is often desirable for electrical connections to a sensor elementcomprising a pattern of electrodes on a substrate to be made along asingle edge of the sensor element. For an orientation in which thisconnection edge is at the bottom of the sensor element, the electrodepatterns will typically comprise electrodes extending generally invertical direction (i.e. to/away from the connection edge) withhorizontal connections between the vertical electrodes to define anappropriate pattern, for example a pattern such as represented in FIG.5. However, as noted above a straightforward configuration of linearelectrodes (e.g. directly following the pattern of FIG. 5) overlaying adisplay screen comprising colour pixels (e.g. as represented in FIG. 6)can give rise to distracting effects. Typical dimensions for a pixel ofa display screen of the kind represented in FIG. 6 might be around 100microns square with the sub-pixels having dimensions of around 33microns×100 microns. A typical copper electrode to be used in atransparent screen might have the width of around 3 microns. Thus, andelectrodes running vertically over the display screen of FIG. 6 willobscure approximately 10% of one colour of sub-pixel. Furthermore,depending on the pitch of the vertical electrodes defining thetouchscreen (i.e. separation between electrodes running generally in thesame direction), adjacent electrodes may obscure different colouredsub-pixels. It is this which can give rise to some of the distractingeffects described above.

In order to address these issues, the Inventor has established newpatterns of electrodes which extend generally in a first direction (e.g.vertical) and may be selectively interconnected along a second direction(e.g. horizontally) and provided with gaps along the first direction todefine a desired electrode pattern for a touchscreen.

FIG. 7 is a diagram schematically representing a portion of a sensorelement 70 comprising electrodes arranged on a network of lines 72 inaccordance with certain embodiments of the invention and an underlyingdisplay screen 60 of the kind represented in FIG. 6. As noted above, thedisplay screen may be based on any conventional display screentechnology. The portion of the sensor element 70 represented in FIG. 6comprises a portion overlying a six-by-six array of pixels 62 in theunderlying display 60. It will be appreciated the sensitive surface ofthe sensor element 70 and the display screen 60 will typically extendover a larger area and contain a larger number of pixels.

In the sensor element 70 of FIG. 7, conductor material is selectivelyarranged along the lines 72 to define electrodes extending generally ina first direction (vertical in the orientation of FIG. 7). Thesevertical lines may be interconnected in a second direction (horizontalin the orientation of FIG. 7) as desired to define an appropriateelectrode pattern. In some respects the arrangement of the network oflines 72 represented in FIG. 7 provides a template on which conductivematerial may be selectively disposed to define electrodes extendinggenerally in a first direction. Whether or not electrodes actuallyfollow some or all of any given one of the lines 72 in a givenimplementations will depend on the underlying electrode pattern to bedefined. The conductive material defining the electrodes may be disposedon a substrate 74 of the sensor element 72 to follow selected parts ofthe network of lines 72 in accordance with any known techniques forproviding conductive material on a substrate, for examplelithographic-based techniques. It is assumed in this example the widthof the individual lines 72 comprising the network of lines is around 3to 5 microns. However, it will be appreciated that other sizes can beselected according to the implementation at hand and having regard toestablished practices of providing electrodes on a substrate. In thisregard, it will be appreciated what is significant for certainembodiments of the invention is not the manner of construction ordeposition of the electrodes (for example in terms of the materials ofthe electrodes and the substrate), or even the general large-scalepattern of electrodes that is desired to provide the touch sensitiveaspects of the sensor element, and these may all be generallyconventional. What is significant in accordance with certain embodimentsis the manner in which electrodes extending in a first direction followlines arranged in a generally zigzag (saw-tooth) pattern instead ofstraight lines, and the specific manner in which the zigzag pattern isarranged, as discussed further below. It is this zigzag configurationthat has been found by the Inventor to provide a way of providingelectrodes extending generally in a first direction to form the basisfor an electrode configuration of a touch sensitive sensor. That is tosay, the zigzag network of lines 72 represented in FIG. 7 provide ageneral template according to which conductive material may be depositedon the substrate 74 to define electrodes running in a verticaldirection, but it will be appreciated that not all portions of thenetwork of lines will be provided with conductive material, rather theconductive material be selectively arranged along the lines to definethe underlying electrode pattern.

Thus the sensor element 70 is for sensing the position of an object in afirst direction (vertical in the orientation of FIG. 7) and a seconddirection (horizontal in the orientation of FIG. 7). For example, theobject may be a finger or stylus place by a user over a relevant part ofthe underlying display 60. The sensor element comprises a substrate 74having an electrode pattern disposed thereon to define an array ofsensor nodes (i.e. regions over which the sensor element is sensitivethe presence of an object. The electrode pattern comprises a pluralityof conductors arranged on a network of lines 72 extending generally inthe first direction. Each of the lines 72 follows a zigzag (saw-tooth)pattern comprising an alternating series of first line segments 76arranged at a first angle (α) to the first direction and second linesegments 78 arranged at a second angle (β) to the first direction. Asignificant aspect of the arrangement of the respective line segmentscomprising the network of lines is that respective ones of the firstline segments in each line are co-linear with respective ones of firstline segments in adjacent lines. The angled nature of the first linesegments with respect to the first direction (the first direction beingparallel to the boundaries between neighbouring sub-pixels in thedisplay) means the electrode material disposed along the lines do notsolely obscure one particular colour of sub-pixel throughout a column asthey would if they simply ran vertically. Furthermore, by having thefirst line segments in neighbouring lines co-aligned with each other,the overall pattern of lines is broadly similar to that of a continuousarray of straight lines extending across the sensor element at an angleα to the first direction. This is an arrangement which the Inventor hasrecognised to have low visibility to a user (i.e. it is an arrangementthat does not give rise to significant visible artefacts), but does notallow for electrodes extending generally in the first direction, whichis what is desired for many electrode patterns, such as the examplearrangement represented in FIG. 5. Accordingly, the zigzag nature of thelines represented in FIG. 7 is provided so that despite being made up ofline segments that are angled with respect to the first direction, thelines nonetheless generally extend in the first direction.

As can also be seen in the arrangement of FIG. 7, respective ones of thesecond line segments 78 in each line are not co-linear with respectiveones of the second line segments 78 in an adjacent line. This is aconfiguration that helps to have the joins between first line segmentsand second line segments in one line at the same locations along thefirst direction as the joins between first line segments and second linesegments in an adjacent line. That is to say, each line matches itsneighbouring line in overall shape but with an offset in the seconddirection orthogonal to the first direction.

In the arrangement of FIG. 7, the extent of a first line segment and asecond line segment along the first direction (i.e. the extent of one“tooth” of the saw-tooth/zigzag along the first direction) correspondswith an integer number of pixels in the vertical direction. In this casethe integer multiple is 2—i.e. each “tooth” spans two pixels. This hasbeen found to reduce visible artefacts associated withelectrode/conductor material selectively disposed on the substrate 74according to the network of lines 72.

Furthermore, the separation of adjacent lines in the second directioncorrespond with an integer multiple of the width of sub-pixels alongthis direction, in this case the integer multiple is 2. More generally,the separation of adjacent lines in the second direction may correspondwith an integer multiple of the width of sub-pixels along the firstdirection does not correspond to the number of sub-pixels in a pixel ofcase (in this case three). This can help ensure that different lines atdifferent locations across the display do not always partially obscurethe same combinations of pixels and this can help to “average out” thevisual impact of conductor material deposited along the lines 72.

In the arrangement of FIG. 7 the zigzag patterning is represented asbeing aligned to the underlying pixel arrangement. For example, each“tooth” shown extending across two complete pixels with the ends of eachtooth arranged at a boundary between two sub-pixels. This arrangement isshown purely to aid an understanding of the relative geometries of thefirst and second line segments with respect to the underlying pixels andsub-pixels in accordance with certain embodiments. In practice, anarrangement of lines having this geometry may be arranged arbitrarily(e.g. in terms of translational offset) with respect to the boundariesbetween the underlying pixels. That is to say, there is no requirementfor the substrate 74 to the arranged over the display 60 with anyparticular degree of precision to obtain the reduced visible artefactsassociated with certain embodiments of the disclosure.

A consequence of the specific arrangement represented in FIG. 7 is thatthe respective first and second line segments are of different lengths,and in particular the first line segments are longer than the secondline segments.

Assuming the pixels 62 in the display 60 are generally square anddivided into three sub-pixels as represented in FIG. 6, and with thescale of the zigzag lines 72 and the separation between neighbouringzigzag lines being as set out in FIG. 7, the angle α at which the firstline segments are inclined to the first direction is around arctan ⅓,which is around 18 degrees. In this particular example the first andsecond line segments are generally orthogonal to each other, andconsequently the angle β at which the second line segments are inclinedto the first direction is around 90—α, which is around 72 degrees in theexample of FIG. 7. However, it will be appreciated that different anglesmay be used for different configurations (e.g. according to the relativesize of each tooth of the zigzag lines and the underlying pixels). Thusthe angle α between respective ones of the first line segments and thefirst direction may, according to different implementations, be lessthan an angle selected from the group comprising: 45 degrees; 40degrees; 35 degrees; 30 degrees; 25 degrees; 20 degrees; 15 degrees and10 degrees. Likewise, the angle β between respective ones of the secondline segments and the first direction may be greater than an angleselected from the group comprising: 45 degrees; 50 degrees; 55 degrees;60 degrees; 65 degrees; 70 degrees; 75 degrees and 80 degrees accordingto the implementation at hand.

It is not necessary for the first and second line segments to beorthogonal, but having an angle of around 90° between the first linesegments and the second line segments has been found to be appropriate.If the angle were to be significantly less than 90° it can be harder toclearly deposit the conductor along the lines without rounding theangle, which may become visible to a user. If the angle were to besignificantly more than 90°, there would need to be a great separationbetween the ends of respective pairs of co-linear first line segments inneighbouring lines, and this may also increase visibility for conductingmaterial deposited in such a way as to selectively follow portions ofthe array of lines 72.

Thus in accordance with certain embodiments of the invention a sensorelement for a touch sensor is provided for use in a touch screen. Thesensitive area of the touch sensor is defined by an arrangement ofconductors deposited on a substrate. The overall/large scale (i.e. ascale on the order of the sensitive area of the sensor element) patternof the electrodes defining the sensing area of the sensor element mayfollow conventional design techniques (e.g. conforming generally to theoverall pattern shown in FIG. 5 in one example), but on a smaller scale(i.e. a scale on the same order as the scale of pixels of the display),the individual sections of conductive material extending generally in afirst direction follow a generally saw-tooth/zigzag pattern. Moregenerally, adjacent electrodes extending in the first direction arearranged so that parallel first line segments of their respectivezigzags are co-linear with corresponding first line segments of thezigzags of neighbouring electrodes. In some respects this approach basedon disposing conductive material on a substrate so as to follow anetwork of zigzag lines to provide electrodes extending generally in afirst direction can be seen as proving an overall design templateaccording to which electrodes are selectively disposed on the substrateto build a desired conductor pattern.

An example of using the principles described herein to provide a portionof a desired conductor/electrode pattern for a touch sensor based on thenetwork of lines represented in FIG. 7 is schematically shown in FIG. 8.

FIG. 8 is a diagram schematically representing a portion of the sensorelement 70 represented in FIG. 7. FIG. 8 is similar to, and will beunderstood from, FIG. 7. However, whereas FIG. 7 shows a portioncorresponding to a six-by-six array of pixels 62 in the underlyingdisplay 60, FIG. 8 shows a slightly smaller portion corresponding to afour-by-four array of pixels 62 in the underlying display 60.Furthermore, FIG. 8 additionally shows an arrangement of a firstelectrode 82 (shown solid in FIG. 8) and a second electrode 84 (showndashed) deposited on the substrate 74 of the sensor element 70. Aspreviously discussed, the electrodes may comprise conductors, forexample made of a thin (e.g. 1 to 10 microns wide) non-transparentconductive material, such as copper, deposited on the substrate 74 to adesired pattern using conventional techniques (such aslithography/printing). The desired pattern will depend on the overalldesign of the sensor element, which may be broadly based on conventionaldesigns, but using zigzag lines as discussed above instead of straightlines.

For the particular example represented in FIG. 8 it is assumed theelectrode pattern provided by the first electrode 82 and the secondelectrode 84 are intended to correspond with a section of an overallpattern providing a single sensor node formed from an interdigitatedseries of conductors based on the general arrangement of electrodesrepresented in the vicinity of the circle 42 shown in FIG. 5 at theintersection between row R1 and column C5. As can be seen in FIG. 8 theelectrode patterning comprising the first and second electrodes 82, 84is arranged to generally follow the underlying pattern of the network oflines 72, but with some additional interconnections between conductormaterial on adjacent lines and with gaps between conductor material on agiven line so as to define the desired pattern (namely an interdigitatedseries of conductors). Some of the interconnections between conductormaterial on adjacent lines 72 are shown within the circles identified byreference numeral 86 and some of the gaps in conductor material on asingle line are shown within the circles identified by reference numeral88. Thus, the arrangement of the first electrode 82 and the secondelectrode 84 represented in FIG. 8 provides a sensor node of the kindidentified by reference numeral 42 in FIG. 5, but based on electrodesthat follow zigzag lines instead of straight lines (aspects ofelectrodes associated with neighbouring sensor nodes are not shown inFIG. 8 for simplicity). The Inventor has recognised this approach canprovides the same functionality as arrangements based on straight-lineconductors, such as shown in FIG. 5, but with reduced visual artefactsassociated with the conductive material providing the electrodesdefining the sensitive area of the sensor element.

In the example of FIG. 8, the various gaps 88 and interconnections 86are relatively small, for example less than 10% or 5% of the length ofthe line segments in which they are provided. This can help reduce theirvisibility. Furthermore, the interconnections 86 are generally providedby extensions of the respective first line segments of one of thenetwork of lines to meet the conductor material deposited on an adjacentone of the network of lines, which further helps reduce theirvisibility. From a manufacturing perspective, it can be helpful for gapsprovided between adjacent section of conductive material on thesubstrate to have at least a minimum size, for example at least 20microns or 30 microns. This can help reduce the likelihood ofshort-circuit failures arising from the manufacturing process and soincrease production yield. This applies for both gaps between conductormaterial arranged on a single line according to a desired electrodepattern (for example the gaps 88 schematically represented in FIG. 8)and the gaps between conductor material arranged on adjacent lines (e.g.the gaps between the co-aligned first line sections 76 of neighbouringlines 72).

It will be appreciated the arrangement shown in FIG. 8 is providedpurely for the sake of example to identify how electrodes may bearranged in accordance with the network of lines 72 represented in FIG.7 to define a desired pattern. For example, whereas the approach adoptedin FIG. 8 provides a sensor node comprising a series of interdigitatedportions of a first electrode 82 and second electrode 84 on a scale offour-by-four pixels, in general it can be expected in practice the sizeof sensor nodes will be larger. An example arrangement of conductivematerial providing electrodes for a larger portion of a sensor accordingto another example is schematically represented in FIG. 9. Moregenerally, an arrangement of conductive material deposited in accordancewith the network of lines 72 and the gaps and interconnectionsassociated therewith may be provided to define an electrode patternbased on conventional design techniques but the using zigzag lineshaving the characteristics described herein instead of straight lines.

As noted above, a desired electrode pattern can be obtained bydepositing conductive material to generally follow an arrangement oflines 72 as discussed above, but with additional gaps andinterconnections provided in accordance with the desired overallelectrode pattern. In some situations there may be areas of a patternwhich comprise a relatively high number of gaps/interconnections, forexample in the vicinity of a sensor node comprising interdigitatedelectrodes, and some areas which have a relatively low number ofgaps/interconnections, for example in the vicinity of an area whichprovides an earth plane. In such situations it may be helpful in somecases to arrange for a relatively uniform distribution ofgaps/interconnections across the surface of the substrate. For example,where there is flexibility in where to interconnect two lines or whereto break (i.e. introduce a gap) into a single line, an approach may betaken to generate a relatively uniform distribution of gaps 88 andinterconnection lines 86 across the surface of the sensor. For example,an earth plane region may be provided by an arrangement of conductormaterial following continuously along the lines 72 in a region of thesubstrate. In principle these lines could be interconnected usingexternal circuitry. However, adjacent lines could also be interconnectedon the substrate, for example using one or more interconnections betweenrespective pairs of adjacent lines. In this regard, if a unit area ofthe sensor pattern away from the earth plane typically has a givennumber of interconnections, the same or similar number ofinterconnections may be provided in a corresponding unit area in theregion of the earth plane. Likewise, although not strictly necessary,additional gaps may be introduced into the electrodes comprising theearth plane (with appropriate interconnections to neighbouring lines toensure overall connectivity by in effect using neighbouring lines tobridge the gaps) to provide for a relatively uniform density of gapswithin the sensor plane. That is to say, interconnections and gaps whichare not strictly required to define a desired arrangement of electrodesfor sensing, may nonetheless be introduced into the pattern to providean arrangement in which there is a relatively uniform density of gapsand/or interconnections in different regions of the sensor surface. Theinventor have recognised this approach can also help reduce visibilityof the pattern. In general, the inventor has found a reasonablecompromise between uniformity and simplicity of design may be achievedfor a target uniformity corresponding to a situation in which if thesensing area is notionally divided into a number or regions, for exampleregions having a size on the order of a hundred microns by a hundredmicrons, e.g. 70 microns by 200 microns, the number of cuts and/orinterconnections in each region is arranged to be less than a givenpercentage, for example less than 20%, away from the average number ofcuts and/or interconnections for the regions.

The sensing element (sensing surface) represented in FIGS. 6 to 9 can beconnected to conventional drive circuitry for establishing the positionof an object adjacent the sensing surface in accordance withconventional techniques such as discussed above with reference to FIGS.1 to 3. Thus the sensing element can be used in a mutual-capacitancemode in which capacitive coupling between the respective firstelectrodes and the second electrodes are measured to identify whichsensing nodes are associated with a change in mutual capacitance causedby a proximate object. The sensing elements can also be used in aself-capacitance mode in which the self-capacitance of the respectiveelectrodes are separately measured to identify which sensing nodes areassociated with a change in mutual capacitance. In this regard it willbe appreciated the interconnection of the electrodes to define rows andcolumns provides a matrix approach which reduces the number of controlchannels required in accordance with conventional touch-sense oftechniques. More generally, it will be appreciated that aspects of thesensor element 70 and its operation to provide position measurementswhich are not described in detail herein may be implemented inaccordance with any conventional techniques.

It will further be appreciated the electrodes and display pixels may bereferred to herein as row (or horizontal) electrodes/pixels and column(or vertical) electrodes/pixels to provide a convenient way ofdistinguishing the different directions along which theelectrodes/pixels extend. These terms are not intended to indicate anyspecific orientation while a sensor is in use. In general the term “row”may be used to refer to electrodes/pixels extending in a horizontaldirection for the orientations represented in the figures while the term“column” may be used to refer to electrodes/pixels extending in avertical direction in the orientations represented in the figures.However, if a sensor in accordance with an embodiment of the inventionis rotated relative to the representation identified in the figures,what are referred to herein as rows will in effect become vertical, andwhat are referred to herein as columns will in effect become horizontal,but it will of course be appreciated this will have no impact on theoperation of the sensor. It will further be appreciated that whilst thedescribed embodiments have assumed a flat regular square array ofdisplay pixels, the same principles apply for non-square arrays.Furthermore, the plane of the sensing surface need not be flat, and thesensing surface may instead be conformed to a three-dimensional surface.In this regard it will be appreciated terms specifying particulardirectional arrangements, such as first direction, second direction, andco-linear/aligned, should be taken to apply within the plane of therelevant surface (for example, line segments which are co-linear on asensing surface, may not be co-linear in an absolute sense in aCartesian 3D space because the sensing surface is curved.)

FIG. 10 schematically shows some components of a touch sensor 1300according to an embodiment of the invention. The sensor 1300 comprises asensing surface 1302, for example in accordance with any of theembodiments of the invention such as discussed above, coupled to acontroller chip 1304. The controller chip 1304 may, for example, be aconventional “off the shelf” controller chip configured to determine theoccurrence of and report a location of a touch using conventionalcapacitive sensing techniques. The sensor 1300 further comprises aprocessor 1306 arranged to receive a reported position estimate from thecontroller 1304 and to convert the reported position estimate to aphysical position estimate in accordance with the above-describetechniques. The processor 1306 may, for example, comprise a suitablyprogrammed general purpose microprocessor, or field programmable gatearray, or an application specific integrated circuit. Furthermore,although presented in FIG. 13 as two separate elements, it will beappreciated the functionality of the controller 1304 and the processor1306 may be provided in a single element, for example, a singlesuitably-programmed microprocessor.

Thus there has been described a touch sensor that comprises a sensorelement for sensing the position of an object over a display screen in afirst direction and in a second direction. The sensor element comprisesa substrate having an electrode pattern disposed thereon to define anarray of sensor nodes. The electrode pattern comprises a plurality ofconductors arranged to follow portion of a network of lines extendinggenerally in the first direction, wherein each of the lines follows azigzag (saw-tooth) pattern comprising an alternating series of firstline segments arranged at a first angle to the first direction andsecond line segments arranged at a second angle to the first direction,and wherein respective ones of the first line segments in each line areco-linear with respective ones of the first line segments in an adjacentline. Such a configuration leads to a reduced visibility of theconductors overlying the display screen.

Further particular and preferred aspects of the present invention areset out in the accompanying independent and dependent claims. It will beappreciated that features of the dependent claims may be combined withfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims.

The invention claimed is:
 1. A sensor element for sensing the positionof an object in a first direction and in a second direction, the sensorelement comprising: a substrate having an electrode pattern disposedthereon, wherein the electrode pattern comprises a plurality ofconductors selectively arranged on a network of lines extendinggenerally in the first direction, wherein each of the lines follows azigzag pattern comprising an alternating series of first line segmentsarranged at a first angle to the first direction and second linesegments that are shorter than the first line segments and arranged at asecond angle to the first direction, and wherein respective ones of thefirst line segments in each line are co-linear in a direction diagonalwith respect to the first direction with respective ones of the firstline segments in an adjacent line and respective ones of the second linesegments in each line are not co-linear in a direction diagonal withrespect to the first direction with respective ones of the second linesegments in an adjacent line, and wherein the ends of the first andsecond line segments are defined by joins between the first and secondline segments.
 2. The sensor element of claim 1, wherein the anglebetween respective ones of the first line segments and the firstdirection is less than an angle selected from the group comprising: 45degrees; 40 degrees; 35 degrees; 30 degrees; 25 degrees; 20 degrees; 15degrees and 10 degrees.
 3. The sensor element of claim 1, wherein theangle between respective ones of the second line segments and the firstdirection is greater than an angle selected from the group comprising:45 degrees; 50 degrees; 55 degrees; 60 degrees; 65 degrees; 70 degrees;75 degrees and 80 degrees.
 4. The sensor element of claim 1, wherein theangle between the first line segments and the second line segments isaround 90 degrees.
 5. The sensor element of claim 1, wherein the joinsinclude a first join between first line segments and second linesegments in one line are at the same locations along the first directionas a second join between first line segments and second line segments inan adjacent line.
 6. The sensor element of claim 1, wherein theelectrode pattern further comprises connector portions arranged toselectively connect conductors arranged on adjacent lines of the networkof lines together.
 7. The sensor element of claim 6, wherein theconnector portions are arranged to have an approximately uniform densityacross the sensor element.
 8. The sensor element of claim 1, wherein theelectrode pattern further comprises gaps arranged to selectivelyseparate conductors arranged on the same line of the network of linesfrom each other.
 9. The sensor element of claim 8, wherein the gaps arearranged to have an approximately uniform density across the sensorelement.
 10. The sensor element of claim 1, further comprising acontroller coupled to respective ones of the conductors and arranged tomeasure changes in a capacitive coupling associated with the conductors.11. The sensor element of claim 10, wherein the controller is furtheroperable to determine the position of an object based on the measuredchanges in the capacitive coupling associated with the conductors.
 12. Atouch screen comprising the sensor element of claim 1, in which theelectrode pattern is arranged over a display screen.
 13. The touchscreen of claim 12, wherein the display screen comprises an array ofpixels arranged in columns extending along the first direction and rowsextending along the second direction, and wherein each pixel isseparated along the second direction into a plurality of sub-pixels forrepresenting different colours of the display screen.
 14. The touchscreen of claim 13, wherein the combined extent of one first linesegment and one second line segment along the first direction is or isapproximately an integer multiple of the extent of one of the pixels inthe first direction.
 15. The touch screen of claim 13, wherein theseparation between adjacent lines along the second direction is or isapproximately an integer multiple of the extent of one of the sub-pixelsin the second direction.
 16. The touch screen of claim 15, wherein theseparation between adjacent lines along the second direction is or isapproximately an integer multiple of the extent of one of the sub-pixelsin the second direction that is different from the number of sub-pixelsin a pixel.
 17. A method of manufacturing a sensor element for sensingthe position of an object in a first direction and in a seconddirection, the method comprising: disposing an electrode pattern on asubstrate, wherein the electrode pattern comprises a plurality ofconductors arranged on a network of lines extending generally in thefirst direction, wherein each of the lines follows a zigzag patterncomprising an alternating series of first line segments arranged at afirst angle to the first direction and second line segments that areshorter than the first line segments and arranged at a second angle tothe first direction, and wherein respective ones of the first linesegments in each line are co-linear in a direction diagonal with respectto the first direction with respective ones of the first line segmentsin an adjacent line and respective ones of the second line segments ineach line are not co-linear in a direction diagonal with respect to thefirst direction with respective ones of the second line segments in anadjacent line, and wherein the ends of the first and second linesegments are defined by joins between the first and second linesegments.